﻿using System;
using System.Text;

namespace Substring
{
    static class Program
    {
        private static string Substring(this StringBuilder sb, int index, int length)
        {
            return sb.ToString().Substring(index, length);
        }

        static void Main()
        {
            StringBuilder sb = new StringBuilder("Looong string.");
            Console.WriteLine(sb.Substring(1, 3));
        }
    }
}
